Method of forming holographic optical elements free of secondary fringes

ABSTRACT

Holographic optical elements relatively free of unwanted, secondary fringes are produced by passing the light beam from a laser through a rotating diffusing plate to generate a beam of light having a very limited coherence length and a spatial coherence which changes over a period of time. A photographic emulsion having a mirror supported on its reverse side is illuminated by the beam and interference occurs between this primary illumination and illumination reflected from the mirror, creating fringes. No other interference fringes are formed because of the lack of coherence between secondary reflections and other rays of the incident beam. The rotation of the diffusion plate time averages out speckle patterns which would otherwise occur. 
     Alternatively, the illuminating beam has a high degree of spatial coherence but its temporal coherence is reduced and varied over a period of time by changing the wavelength of a tunable-dye laser.

This is a continuation of U.S. Pat. application Ser. No. 927,341, filedNov. 4, 1986, now abandoned, which is a continuation of U.S. Pat.application Ser. No. 613,901, filed May 24, 1984, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to holographic optical elements and methods offorming such elements in such a manner as to make them free of unwantedsecondary fringes by use of light beams having short coherence pathlengths.

2. Prior Art

Holographic optical elements (HOEs) comprise plates having finefringe-like structures which are formed by photographic processesinvolving the recording of interference fringe patterns of two mutuallycoherent light beams. In use, the fringe-like structures diffract lightwavefronts incident on the HOEs to act as lenses, filters and the like.

When HOEs are formed by illuminating a photographic emulsion coated on asupporting plate with a pair of coherent light beams to form aninterference pattern, it is difficult to prevent various secondaryreflections of the incident light beam from interfering to produceunwanted, secondary fringe patterns of a lower intensity than theprimary fringes. One source of these secondary reflections is thereflection that occurs when the primary interfering beam, after havingpassed through the photographic emulsion, enters the immediatelycontacting media, be it air, a liquid or solid material. Despite thefact that the contacting media is transparent, because of thedifferences in indices of refraction between the emulsion and thismedia, some reflections back into the emulsion will occur from theinterfaces. These reflections can be minimized using index-matchingtechniques and anti-reflection coatings, but cannot be completelyeliminated. If the primary light beam has sufficient coherence pathlength, these reflections back into the emulsion will interfere with theprimary beam to produce secondary fringes. In many applications of HOEs,the resulting weak, secondary fringe systems will not deleteriouslyaffect the performance of the device, but in certain applications suchsecondary fringes are highly undesirable. For example, holographicoptical elements are used as combiners in head-up display systems inwhich they act to superimpose instrument displays on the pilot's viewthrough the aircraft windshield. In this application secondary fringesystems may produce undesirable images to the pilot which may interferewith his perception of both the instrument displays and the view throughthe windshield.

SUMMARY OF THE INVENTION

The present invention is accordingly directed toward a method of formingHOEs in such a manner as to achieve a desired fringe pattern without thegeneration of any undesirable secondary fringes.

The present invention preferably achieves this object by producing anHOE from the interference of two light beams having an extremely limitedcoherence path length so that they will form extremely localized fringepatterns but their secondary reflections will be incoherent with oneanother and with the primary incident beams so that no stationaryinterference or resulting fringes will be produced within thephotographic emulsion.

The limited coherence beams used in the present invention have a highdegree of either spatial or temporal coherence and a low degree of theother coherence. Thus, when the path length between the two beams isshorter than the coherence path length of the light source, the beamswill produce higher contrast stationary fringes at their intersection.If the path lengths between the primary beams and their reflections arelonger than the coherence path length, secondary fringe patterns cannotbe produced because the secondary reflections become incoherent withrespect to the primary beams. The two beam sections are arranged so thatthey intersect one another, within the media, in the very shortcoherence path length of the beams. Any reflections produced by thebeams as they exit the media will be incoherent with respect to othersections of the primary incident beams and accordingly, will not producesecondary fringes when they intersect such beams.

The limited coherence beam of the present invention may be produced bypassing light from a laser having a high degree of both temporal andspatial coherence through a random phase (diffusing) plate which lowersthe spatial coherence of the beam but does not appreciably reduce thetemporal coherence. If the phase plate is stationary during the time ofthe exposure, stationary interference patterns would occur at randomlocations between various sections of the beam which illuminate themedia, producing what is termed a speckle pattern. This possibility iseliminated by moving the random phase plate during the exposure time sothat any random interferences are time-averaged to zero, eliminating thespeckle pattern.

Alternatively, the beam could be produced with low temporal coherenceand high spatial coherence by tuning the spectrum of a tunable-dye-laserover the period of the exposure.

One technique for obtaining localized interference in the photographicemulsion with a limited coherence light beam, involves backing up thephotographic emulsion with a mirror and illuminating the emulsion with asingle limited coherence beam. The mirror is in close contact to theemulsion and its reflections are highly coherent with the incident beamand strongly interfere with them. The single beam illumination techniqueusing a mirror has been employed in the prior art to form white lightholograms using beams that are both spatially and temporally coherent.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objectives, advantages and applications of the present inventionwill be made apparent by the following detailed description of twoalternative embodiments of the invention. The description makesreference to the accompanying which:

FIG. 1 is a diagrammatic illustration of the interference patterns whichoccur when a photographic emulsion layer, backed by a mirror, isilluminated with coherent light;

FIG. 2 is a diagrammatic illustration of a first method of practice ofthe present invention wherein a single beam of light of limited spatialcoherence illuminates a photographic emulsion layer backed by a mirrorto form a reflection HOE;

FIG. 3 is a schematic illustration of a first form of spatial coherencecontrol device for use with the present invention, comprising a rotatingrandom phase plate;

FIG. 4 is a schematic diagram of a form of a temporal coherence controldevice for use with the present invention, comprising a tunable-dyelaser;

FIG. 5 is a schematic diagram of a method for practice of the presentinvention using two beams to form a transmission hologram; and

FIG. 6 is a schematic diagram of a method of the practice of the presentinvention using two beams to form a reflection hologram.

DETAILED DESCRIPTION

FIG. 1 diagrammatically illustrates the manner in which undesiredsecondary fringe patterns are generated in the prior art method offorming a reflection HOE using a single coherent beam technique.

A glass plate 10 is coated with a photosensitive layer 12 of the typeused to form holograms, preferably a photographic emulsion.Alternatively other photosensitive materials such as thermoplastics orphotoresists may be used. A planar mirror 14 is placed with itsreflective surface in close proximity to the emulsion layer 12 by anindex matching medium 16.

The glass plate 10 is illuminated by a beam 18 of coherent light,derived from a laser 19 and a beam forming optical system 25, which isincident on the plate. One ray R1 of the beam 18 will be incident on theglass plate 10, at an angle of θ to the normal of the photosensitiveemulsion 12 and, the index matching medium 16 and will be reflected bythe mirror surface 14 back through the layers at a complementary angleof θ to the normal in the form of a ray R2. As the wavefront of the rayR2 passes through the emulsion 12 in a direction opposite to thewavefronts of R1, stationary interference patterns will occur which aresubstantially constant over the period of the exposure. The differencein path length between these two beams is very short, typically on theorder of 30 microns to several centimeters. The stationary pattern ofconstructive and destructive interference between these two beamsproduces latent images in the photosensitive emulsion in the form of afringe pattern. Upon subsequent photographic development of theemulsion, a fringe pattern of varying optical transmissivity asrefractive index is produced.

As the reflected ray R2 passes through the interface between the glassplate 10 and the ambient media 20, typically air, a reflected ray R3 isproduced because of the difference between the indices of refraction ofthe two medias. Anti-reflective coatings may be used to minimize thereflection but inevitably some reflection will occur. This secondarilyreflected ray R3 will move in the same direction as another section ofthe primary incident ray R4, and as both move through the emulsion layer12 a stationary pattern of interference will occur if the difference inpath lengths that the two wavefronts travel through to reach this pointis within the coherence length of the incident laser beam 18. Measuringfrom a common wave front point 21 on the two incident beam rays R1 andR4, the difference dp in the path length between the secondary reflectedray R3 and the ray R4 will be approximately S₁ +S₂ +S₃ +S₄ -S₅ -S₆. Ifthis difference is sufficiently greater than the coherence path lengthof the laser, no stationary interference fringes will be produced. Ifthis difference is not greater than the coherence path length of thelaser, secondary interference fringes will be formed. In any event, theinterference fringes produced between this secondarily reflected beamand the primary beam will be weaker in amplitude than the fringesproduced by the primary beam's interference with its own reflection fromthe mirror 14, but these weak secondary fringes may deleteriously affectthe performance of the resulting HOE.

A preferred embodiment of the present invention used to form reflectionHOEs with a single beam technique is diagrammatically illustrated inFIG. 2. A laser light 24 generated by laser 19 is passed through acoherence control device 27 so that the output beam 29 from thecoherence control device which is subsequently passed through beamforming optical system 25, will have a coherence path length L muchshorter than the beam path length difference dp (=S₁ +S₂ +S₃ +S₄ -S₅-S₆). In this case, since the path length difference dp is much longerthan the coherence length L of the beam there are not stationary fringepatterns formed between the secondary reflective ray R3 and the primaryray R4. In practice of the method illustrated in FIG. 2, proper balanceshould be achieved between the path length difference dp and thecoherence path length L of the light source so that a high contrastprimary interference pattern is formed but no secondary interferenceoccurs.

A preferred embodiment of the spatial coherence control device 27 isdiagrammatically illustrated in FIG. 3. A beam 24 from the laser 19 ispassed through beam shaping optics 26 such as a microscope objective tofocus the beam at a plane 28. A circular random phase disk 30 of etchedor ground glass is rotated in its plane by a motor 32 so that the beamintersects the plate beyond the focal plane 28, thus illuminating anarea on the disk. In alternative embodiments the phase plate could bemoved randomly or reciprocated linearly. Because of the variations inthe thickness of the rotating, random phase plate, the rays of the beampassing through different points of the phase plate will have differentphases. Moreover, because of the rotation of the phase plate 30, thephases of any two points on the exiting beam will vary relative to oneanother during the period of the exposure. Accordingly, the spatialcoherence of the beam that exits the rotating random phase plate will bereduced.

In this manner the coherence length L of the exiting beam will bedecreasing at an angular source size θ_(s) 35, which is observed as therecording plane 31, increases. The coherence path length of the beam isapproximately in an inverse relationship to the angular source sizeθ_(s) 35.

A preferred embodiment of the temporal coherence control device isdiagrammatically illustrated in FIG. 4. A host laser 41 providing itsoutput to a tunable-dye laser 40 is operative to generate a collimatedoutput beam 24. A suitable tunable-dye laser is Coherent RadiationCorporation, Model CR599-01. Using a coumarin-7 dye, the output of thelaser may be varied between 505 and 565 nanometers. A bandwidthcontroller 42 is connected to the laser to vary its output during theperiod of the exposure. The nature of the bandwidth controller dependsupon the specific tunable-dye laser being used and may constitute adrive motor which varies the position of the cavity mirror during theperiod of the exposure or alternatively may simply provide a varyingelectrical signal which changes the output of the laser.

The coherence length L of the laser beam is inversely proportional tothe spectral bandwidth of the laser and if the spectral bandwidth of afixed output laser is sufficiently broad it may be unnecessary to changethe tuning of the laser during the exposure time and accordingly theneed for a tunable-dye laser is eliminated.

The output beam 29 of the temporal coherence control of FIG. 4 is usedin the manner illustrated in FIG. 2 to form an HOE without secondaryfringes.

FIG. 5 illustrates the manner in which a limited coherence beam, such asthe type produced by the systems of FIGS. 3 or 4, is used to form atransmission HOE. The output beam from a laser 19 is passed through aspacial or temporal coherence control device 27 to produce a limitedcoherence output beam 29 with coherence length L. A beam forming opticalsystem 50, such as a microscope objective, pinhole, or the like providesa collimated output beam which is incident upon a beam splitter 52. Onebeam section is reflected by a mirror 54 through another suitable beamforming optical system 58 and the output beam is incident upon aphotosensitive emulsion 62 formed on a substrate 64.

The other beam from the splitter 52 is reflected by a mirror 56 througha beam forming optical system 60 and the output beam is incident uponthe emulsion 62 at an angle relative to the first beam. The path lengthsof the two beams from the beam splitter to the substrate are carefullycontrolled so as to be substantially identical. Accordingly, when thetwo beams intersect within the emulsion stationary interference patternsare formed during the period of the exposure despite variations in theinput beam that may be produced by the coherence control unit 27.Reflections which occur when the incident beams reach the interfacebetween the substrate 64 and the ambient media 66 are relativelyincoherent with respect to the primary incident beams so that nostationary fringe patterns are formed when they intersect with thesebeams within the emulsion.

The photosensitive emulsion is then developed by suitable means toprovide the desired HOE.

FIG. 6 illustrates essentially the same apparatus used to form areflection HOE. The emulsion 62 and supporting substrate are supportedrelative to the two limited coherence beams, of equal path length, sothat the beams enter the emulsion from opposite sides. A cover plate 70of substantially the same constitution as the substrate 64 matches thepath length of the two beams to compensate for the refraction thatoccurs by passage of one of the beams through the substrate 64. A indexmatching medium 68 interfaces the abutting surface of the cover plate 68and the emulsion 62.

I claim
 1. The method of constructing a reflection holographic opticalelement free of secondary interference fringes comprising:forming aplanar transparent supporting substrate having a predetermined thicknessand first and second surfaces; disposing a layer of transparentphotosensitive material on said second surface of said transparentsupporting substrate, said layer of photosensitive material having afirst surface in contact with said second surface of said transparentsupporting substrate and a second surface; forming a first light beamfrom a polychromatic point source having a predetermined spectralbandwidth, said predetermined spectral bandwidth forming an effectivecoherence length inversely proportional thereto; illuminating saidtransparent supporting substrate and said layer of photosensitivematerial through said first surface of said transparent supportingsubstrate with said first light beam; reflecting said first light beamafter it passes through said layer of photosensitive material from areflecting surface disposed in immediate proximity to said secondsurface of said layer of photosensitive material, thereby forming asecond light beam which passes through said layer of photosensitivematerial and said transparent supporting substrate; and developing saidlayer of photosensitive material;whereby said predetermined spectralbandwidth of said first beam is selected having an effective coherencelength greater than the distance along the light path from said firstsurface of said photosensitive material to said reflecting surface andreflected to said first surface of said photosensitive material suchthat the intersection of said first light beam and said second lightbeam forms interference fringes of high contrast throughout said layerof photosensitive material and said predetermined spectral bandwidth ofsaid first light beam is selected having an effective coherence lengthless than the distance along the light path from said second surface ofsaid photosensitive material to said reflecting surface, reflected tosaid first surface of said supporting substrate and further reflected tosaid second surface of said photosensitive material such that theintersection of said first light beam and a third light beam formed bythe reflection of said second light beam from said first surface of saidtransparent supporting substrate caused by the difference in index ofrefraction between said transparent supporting substrate and the ambientmedia beyond said first surface of said transparent supporting substratedoes not form interference fringes of high contrast anywhere within saidlayer of photosensitive material.
 2. The method o constructing areflection holographic optical element as claimed in claim 1wherein:said step of forming said first light beam consists ofgenerating light from a tunable-dye laser having a wavelength changeduring said step of illuminating for producing said predeterminedspectal bandwidth.
 3. The method of constructing a reflectionholographic optical element as claimed in claim 1, wherein:said step ofreflecting said first light beam consists of disposing a reflectingsurface in close proximity to said second surface of said layer ofphotosensitive material and filling the space between said layer ofphotosensitive material and said reflecting surface with an indexmatching fluid having an index of refraction substantially equal to theindex of refraction of said layer of photosensitive material.
 4. Themethod of constructing a reflection holographic optical element free ofsecondary interference fringes comprising:forming a planar transparentsupporting substrate having a predetermined thickness and first andsecond surface; disposing a layer of photosensitive material on saidsecond surface of said transparent supporting substrate, said layer ofphotosensitive material having a first surface in contact with saidsecond surface of said transparent supporting substrate and a secondsurface; forming a first light beam from a monochromatic point source;forming a second light beam from said first light beam by forming anextended source of independently radiating elements having apredetermined angular source size from said first light beam, saidpredetermined angular source size forming an effective coherence lengthinversely proportional thereto; illuminating said transparent supportingsubstrate and said layer of photosensitive material through said firstsurface of said transparent supporting substrate with said second lightbeam at a predetermined illuminating angle; reflecting said second lightbeam after it passes through said layer of photosensitive material froma reflecting surface disposed in immediate proximity to said secondsurface of said layer of photosensitive material, thereby forming asecond light beam which passes through said layer of photosensitivematerial and said transparent supporting substrate; and developing saidlayer of photosensitive material;whereby said predetermined angularsource size of extended source of independently radiating points of saidsecond light beam is selected with respect to said predeterminedilluminating angle having an effective coherence length greater than thedistance along the light path from said first surface of saidphotosensitive material to said reflecting surface and reflected to saidfirst surface of said photosensitive material such that the intersectionof said second light beam and said third light beam forms interferencefringes of high contrast throughout said layer of photosensitivematerial and is selected with respect to said predetermined illuminatingangle having an effective coherence length less than the distance alongthe light path from said second surface of said photosensitive materialto said reflecting surface, reflected to said first surface of saidsupporting substrate and further reflected to said second surface ofsaid photosensitive material such that the intersection of said secondlight beam and a fourth light beam formed by the reflection of saidthird light beam from said first surface of said transparent supportingsubstrate caused by the difference in index of refraction between saidtransparent supporting substrate and the ambient media beyond said firstsurface of said transparent supporting substrate does not forminterference fringes of high contrast anywhere within said layer ofphotosensitive material.
 5. The method of constructing a reflectionholographic optical element as claimed in claim 4, wherein:said step offorming a second light beam from said first light beam consists ofilluminating a first surface of a diffuser plate with diverging lightfrom said monochromatic point source producing said predeterminedangular source size at a second surface of said diffuser plate, saidpredetermined angular source size determined by the distance betweensaid monochromatic point source and said first surface of said diffuserplate, and moving said diffuser plate thereby forming said extendedsource of independently radiating elements.
 6. The method ofconstructing a reflection holographic optical element as claimed inclaim 4, wherein:said step of reflecting said second light beam consistsof disposing a reflecting surface in close proximity to said secondsurface of said layer of photosensitive material and filling the spacebetween said layer of photosensitive material and said reflectingsurface with an index matching fluid having an index of refractionsubstantially equal to the index of refraction of said layer ofphotosensitive material.